Receiver loop circuit



July 29, 1941. D. E. FOSTER RECEIVER LOOP CIRCUIT Filed April 27, 1940 A. F. AMPLIFIER CONVERTER 70 AVC 8/45 SOURCE INVENTOR. DUDLEY FOSTER BY ,5

ATTORNEY,

Patented July 29, 1941 RECEIVER LOO-P CIRCUIT Dudley E. Foster. South Orange, N.- J assignor to Radio Corporation of America, a corporation of Delaware Application April 27, 1940, Serial No. 331,912

6 Claims.

My present invention relates to loop antenna circuits for radio receivers, and more particularly to a tunable loop circuit for a radio broadcast receiver.

The loop type of signal collector for radio broadcast receivers is being widely used at the present time. Besides delivering a'high ratio of signal voltage to noise voltage to the first transmission tube of the receiver it isdesirable that the loop circuit in a superheterodyne receiver transmit no image frequencies, or energy of intermediate frequency (1. F.). In any superheterodyne receiver the two most serious types of spurious responses are the image and I. F. responses. In receivers having a' primary coil on the input transformer, combined capacitance and mutual inductance coupling in opposing sense are frequently used to improve rejection'of image response. The capacitiveand inductive couplings are made equal and-opposite at image frequency so no appreciable energy transfer exists at that frequency, whereas, because of difference in rate of variation with frequency substantial coupling exists at the desired frequency. In a loop receiver there is no primary, the voltage being induced directly into the tuned circuit, so that this image cancellation effect cannot be realized. For I. F. rejection in receivers designed for anexternal antenna, trap circuits are used. Trap circuits are not as conveniently applied to loop antennae, but this invention points out how such an I. F. rejection circuit may be conveniently applied with a minimum number of'components. Further, the loop circuit should be of minimum damping so as to be highly eificient in transmitting signal voltage to the first tube of the receiver system.

It may, accordingly, be stated thatit is one of the main objects of my present invention to provide a loop antenna circuit for a receiver wherein the circuit as a whole is tunable over a desired signal frequency range, but a branch thereof is simultaneously tunable over a range of undesired image frequencies whereby energy of image frequency is rejected at the input electrode of the first signal transmission tube.

Another important object of the invention may be stated to reside in the provision of a tunable loop circuit for a radio broadcast receiver of the superheterodyne type, wherein the loop circuit includes a branch thereof which is fixedly resonated substantially to the operating I. F. value of the receiving system whereby the loop circuit is capable of rejecting incoming signal energy of the I. F. value thereby eliminating a normally-vexatious source of interference.

Another object of my invention is to provide in association with the input electrodes of the first tube of a broadcast receiver a tunable loop antenna circuit which is regenerative, thereby to minimize damping, by virtue of a signal feedback circuit between the output electrodes of the first tube and its input electrodes. I

Still another object of myiinvention is toprovide in a loop antenna circuit for a radio receiver a condenser element which is arranged in series in the loop circuit with the inductive and condenser elementsof the latter whereby the condenser'element functions simultaneously as a direct current blocking condensen as'a regenerative feedback path between the outputand input electrodes of the first receiver tube and as a part of an I. F. interference rejection network. r T V V Still other objects of the inventionareto improve generally. the. simplicity and efficiency of signal collector circuits for radio'receivers, and more especially to provide a signal collector of the tunable loop type for a vradio broadcast receiver and which loop circuit is not only reliable and efiicient in operation, but is economically manufactured andassembled in a radio receiver.

The novel features which I believe to be characteristic of my invention are set forth in par ticularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into eifect. a Y

Referring now to the accompanying drawing, there is shown that portion of a superheterodyne broadcast receiver which is pertinent to a proper understanding of this invention. Let it be understood that the receiver is capable of operating in the broadcast range of 550 to 1600 kilocycles (kc), and that it is provided with a loop antenna which includes a coil L and a variable tuning condenser C. It is not believed necessary to show the construction of the loop antenna circuit indetail since those skilled in the art are fully aware of its construction. The coil L-particularly will comprise a plurality ofwindings arranged in a, square formation as is well known.

The condenser C is adjustable to tune the 'coil L over the range of broadcast frequencies, and the condenser rotor itself is ganged for unicontrol with the rotors of the remaining tuning condensers of a superheterodyne receiver. Assuming that the first tube of the receiver is a radio frequency amplifier, the tube I has its cathode established at ground potential, and the grounded cathode is connected to the low potential end of coil L through a condenser C1. The input grid 2 of tube l is connected to a predetermined tap on coil L, and the tap point is denoted by the numeral 3.

The tap on the coil divides the latter into a portion L1 and a larger section In. The con; denser C1 is arranged in series with the coil L and condenser C. The output electrode,or plate, of tube l is connected to a sourceof positive potential (not shown to preserve simplicity of disclosure) through the primary winding P of the radio frequency transformer T. The sec ondary winding S is tuned by the variable con-' denser 4 over thedesired signal frequency range. The q i d l .5 da ces the mebhani 1m control device between the rotors' of the variable condensers C and 4. A regenerative feedback path for the modulated fsi gnalicarrier energy is arranged between the plate, of tube l and the lowpotential end of the loopcircuit coil L. The feedback path comprises condenser C2 in series with the condenser C1.

The tunable circuit 4 may reed the input electrodes of the usual converter, or first detector, tube, and; if desired, the 'converter may be of the combined local o'scillator first detector type. .Qffcourse, thelocal oscillator may be a separate tunable v oscillator circuit, and would in that ase be tunable over a, range of oscillation f equ n hi h ,t nihe s na fr u n range but constantly differing therefrom by the predetermined I. F value- The latter may be chosen from a range of 75 to about 455 kc; It is not believed necessary to show these various details Y or a. superheterodyne receiver, since those skilled in theart, are fully aware of the construction of such a receiver In general, the I. F. o utv i n yb the w y ne .w i dL e e t rbu h one or more I. F.,amplifiers, andthe amplified L F energ'y wouldthen be fed to ai s'econdfdee te i ea e' ted' mq' a 'i n ntag would bag fli 9 in 9 mo e. aud o pl fie and finally be reproduced in any loudspeaker, Thereceiver is usually equipped withautornatic volume controlfiAVC) and such a contr olcircuit y. plgys a rectifier which is coupled to the IF. network and produces adirectcurrent voltage whose value is substantially proportional to the I. carrier amplitude. I s s V i-lence the low potential end of coil L is shown connected to a direct current voltage lead H) which includes the pulsation voltage filter I I. It will be understood that the lead It) will be con necte'd to the AVC sa s-three, andwhere the rectifier is dent diode type the connection It) is made to the diode anode end of therectifier load resistor, There may be provided a normal negative bias voltage over lead l of approximately z 0: atoms so that the signal rid 2 will have a normal negative po-s ai bias with respectto the grounded cathode. As stated previously these various circuits are too well known ,to those stones in the art to need any inustranon, or to reqgire any further description. t

The magnitudes of the inductive section L1 and the variable condenser Care chosen so that Li -C is approximately resonant to the undesired image frequency at the respectively different station settings of the tuning mechanism 5. In

other words, L1-C will be tunable approximately over an undesired image frequency range. As is well known, this image frequency range is that frequency range which is as much above the local oscillator frequency range as the signal frequency range is below the oscillator frequency range. In other words, the image frequency range differs from the oscillator frequency range by the predetermined I. F. value. The resonant branch L1C serves in effect as an image frequency rejection network. In association with the loop antenna circuit such an image rejection branchis of great utility, and enhances the chiciency of the, loop circuit to a great extent.

For the purposes of simplicity of explanation, let us assume that the mutual inductance between portions L1 and Le of the inductance is negligibly small. In practice it may be negligibly small, or may have appreciable magnitude as may be most convenient. Withthe mutual coupling negligibly small, L the total inductance equals L1 plus Li. L" resonated to the desired signal by the series combination of C and C1. Let us assume that the I. F. is 455 kc, andthe incoming signal is 1000 kc. The oscillator is then 1455 kc., anda signal at 1910 kc; (the image frequency) will also produce 455 kc, and pass through the I. F. network. If Li'-C be made resonant to 1910 k'c., this circuit produces a very low impedance between grid and cathode of the amplifier tube 1, substantially preventing amplification of energy at that frequency. Now, if'the'desired signal be 600 kc., the oscillator becomes 1055 kc., and the image frequency'15l0 kc. To tune the loop to the desired signal, from 1000 to 600 kc., or change of 1.67 to 1, requires a capacitance change of the square of that ratio or 2.77 to 1. Assuming 'for the moment that Ci' is'so large as to have no appreciable effect on the setting of C for resonance, this means a change'in' C of 2.77 to 1'. While the desired signal has changed 1.67 to 1 the image has changed only from 1910 kc. to 1510 kcl, or 1.26 to 1. Hence, L1--'C is not precisely resonant to the image frequency at the new (600 kc.) de-' sired frequency. However, it is partially resonant, and hence improves the image ratio, but not to the maximum degree. The size of L1, that is the portion of the loop to the grid .tap, may be chosen togive maximum image ratio at any desired frequency. As has been shown, L1-C will not be precisely resonant at other frequencies, but will nevertheless substantially improve the image ratio at all frequencies over what would exist if the grid were not tapped down on the loop.

-In order to prevent reception of signal ire quencies whose carrier is of substantially the value of the predetermined I. F., the inductive section L2 is resonated by "condenser C1 to the I. F. value. Hence, 142-01 serves asan I. F.r'e-' jection branch. Here, again, the inclusion of this rejection branch in the loop circuit greatly enhances the utility and operating efficiency of the loop circuit. Just as circuit L1 C acts as a low impedance between grid and cathode at image frequency, so Lz--'C1 may be made, by proper choice of C1, resonant-to the intermediate fre quency; Circuit Lz-C1' then acts as a low impedance between grid and cathode for the intermediate frequency. Since the I. F. is a constant, L2 and C1 may be fixed in value. Since the I. F. is lower than the desired frequency, and since L2 is smaller than L1 plus L2, C1 will be substantially larger than the maximum value of C. t e The 'co'ndenser Cr functions, additiqnally, as part of the regenerative feedback path which includes the condenser C2. This enables the damping of the loop circuit as a whole to be minimized thereby increasing selectivity and voltage step-up of the loop circuit. By connecting the capacitor C2 in the plate of the amplifier tube to the junction of L and C1 in place of ground, C1 is included in both the plate-cathode and grid-cathode paths and, therefore, produces some feedback. The feedback is not sufficient to cause oscillation but does tend to cancel some of the radio frequency resistance of the loop, thus improving the selectivity and gain characteristics of the loop circuit. The value of C1 is determined by the I. F. rejection requirement; since this capacitance also provides the feedback, it may be desired to change the feedback without changing the I. F. rejection. This may be done by using two capacitances in series for C1, and connecting C2 to the junction of the two capacitors. If the two capacitors in series have the same total capacitance as C1 the I. F. rejection remains unchanged, but the regenerative feedback is reduced.

Of course, the entire loop circuit, that is LC1--C tunes as a whole to the desired signal carrier frequency. The condenser C1, additional- 1y, functions as a direct current voltage blocking condenser, and it is for that reason interposed between the lead I and the grounded end of the condenser C. It will, therefore, be appreciated that the condenser 01 performs a triple function in the network since it not only is included in the regenerative feedback path, but it acts as a direct current blocking condenser and also functions to resonate the inductive section L2 to the I. F.

value. It is to be understood that the loop could readily operate directly into the input electrodes of the converter tube.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a loop antenna circuit of the type comprising a coil and tuning condenser, a signal transmission tube having input electrodes and output electrodes, an auxiliary condenser arranged in series with the coil and tuning condenser in said loop circuit, connections between the input electrodes of the tube and a section of the loop circuit coil such that the section of the coil included between the input electrodes resonates with said auxiliary condenser to an undesired interfering signal frequency lower than the desired signal frequency, and the remaining section of said coil resonating with said tuning condenser to an undesired interfering signal frequency which is higher than said desired signal frequency.

2. In a loop antenna circuit of the type comprising a coil and tuning condenser, a signal transmission tube having input electrodes and output electrodes, an auxiliary condenser in said loop circuit, connections between the input electrodes of the tube and a section of the loop circuit coil such that the section of the coil included between the input electrodes resonates with said auxiliary condenser to an undesired interfering signal frequency lower than the desired signal frequency, and a regenerative feedback path between the output electrodes and input electrodes of said tube, said path including said auxiliary cuit coil such that the section of the coil included between the input electrodes resonates With said auxiliary condenser to an undesired interfering signal frequency, a regenerative signal voltage feedback path between said output electrodes and input electrodes and including said auxiliary condenser as an element thereof, a direct current biasing voltage connection to said coil, and said auxiliary condenser being arranged between said connection and the low potential end of said tuning condenser whereby the auxiliary condenser functions as a direct current blocking condenser.

4. In a superheterodyne receiver of the type including a tunable loop antenna circuit, said receiver having a signal transmission tube including at least a cathode, a signal grid and an output electrode, means coupled to said output electrode adapted to transmit the signals passing through the tube to a network tuned to a predetermined intermediate frequency signal, said loop circuit including a coil and a variable tuning condenser adapted to tune said coil over a range of desired signal frequencies, means establishing the cathode of said tube at the same relatively fixed alternating potential as the low potential terminal of said tuning condenser, means connecting said signal grid to an intermediate tap of said coil, said tap being so chosen that said tuning condenser resonates the section of the coil above the tap point over substantially an undesired range of image frequencies, an auxiliary condenser included in series in said loop circuit between said coil and tuning condenser, said auxiliary condenser resonating the remaining section of solely saidtapped coil to the intermediate frequency whereby undesired interference of intermediate frequency is rejected.

5. In a superheterodyne receiver of the type including a tunable loop antenna circuit, said receiver having a signal transmission tube inc1uding at least a cathode, a signal grid and an output electrode, means coupled to said output electrode adapted to transmit the signals passing through the tube to a network tuned to a predetermined intermediate frequency signal, said loop circuit including a coil and a variable tuning condenser adapted to tune said coil over a range of desired signal frequencies, means establishing the cathode of said tube at the same relatively fixed alternating potential as the low potential terminal of said tuning condenser, a direct current blocking condenser in series with said coil and tuning condenser at the low potential side of the loop circuit means connecting said signal grid to an intermediate tap of said coil,means applying a direct current bias voltage to said grid through said coil, said tap being so chosen that said tuning condenser resonates the section of the coil above the tap point over substantially an undesired range of image frequencies, and a regenerative feedback path including said blocking condenser connected between said output electrode and said loop circuit for minimizing the damping of the latter.

6. In a superheterodyne receiver of the type including a tunable loop antenna circuit, said receiver having a signal transmission tube in cluding at least a cathode, a signal grid and an output electrode, means coupled to said output electrode adapted to transmit the signals passing through the tube to a net-work tuned to a predetermined intermediate frequency signal, said loop circuit including a coil and a variable tuning condenser adapted to tune said coil over a range of desired signal frequencies, means establishing the cathode of said tube at the same relatively fixed alternating potential as the low potential terminal of said tuning condenser, means connecting said signal grid to an intermediate tap of said coil, said tap being so chosen that said 

